Aqueous solution containing a resin and sucrose for use with solutions of hexavalentchromium coating compounds



United States Patent AQUEOUS SOLUTION CONTAINING A RESIN AND SUCROSE FOR USE WITH SOLUTIONS OF HEXAVALENT CHROMIUM COATING COMPOUNDS Ludwig K. Schuster, Dresher, and Alfonso L. Baldi,

Jr., Drexel Hill, Pa., assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., :1 corporation of Pennsylvania No Drawing. Filed May 23, 1963, Ser. No. 282,571

3 Claims. (Cl. 26017.4)

The present application is in part a continuation of prior applications Ser. No. 666,852 filed June 20, 1957 (now U.S. Patent 3,112,231 granted Nov. 26, 1963), Ser. No. 6,021 filed Feb. 1, 1960, (now U.S. Patent 3,128,546 gnanted April 14, 1964) and Ser. No. 88,018 filed Feb. 9, 1961 (now U.S. Patent 3,094,435 granted June 18, 1963). Ser. No. 6,021 is in turn a continuation-in-part of Ser. No. 592,552 filed June 20, 1956 (now U.S. Patent 3,031,333 granted Apr. 24, 1962), Ser. No. 708,772 filed Jan. 14, 1958 (now abandoned), Ser. No. 738,648 filed May 29, 1958 (now U.S. Patent 3,053,693 granted Sept. 11, 1962) and Ser. No. 814,200 filed May 19, 1959 (now U.S. Patent 3,053,702 granted Sept. 11, 1962). Ser. No. 88,018 is al-so a continuation-in-part of Ser. No. 708,772.

This invention relates to the coating of metal, more particularly to improve the corrosion resistance of the metal as in the manufacture of storage and shipping containers, building panels, and any other metal articles that are subject to corrosion.

Among the objects of the present invention is the provision of novel compositions for preparing the above coatings as well as novel methods for applying them.

The above as well as additional objects of the present invention will be more fully understood from the following description of several of its exemplifications.

The above prior applications describe the coating of corrodible metals, particularly plain carbon steels such as SAE 1010 steel with an in situ formed combination of hydrated chromium oxides containing about 20 to 60% chromium by weight, about 40 to 95% of the chromium by weight being trivalent, the remainder being hexavalent, and the coating weighing about to 200 milligrams per square foot, and in some cases as little as one milligram per square foot, of surface that it covers. As disclosed in application Ser. No. 592,552, such plain carbon steels, if given a grain-boundary etch before coating, show much greater corrosion resistance.

A can for liquid detergent, lubricating oil, coffee beans, or the like, can have its inner surface covered with the above coating to prevent rust prior to filling as well as after filling with the above materials; the outer surfaces of the can also have the same type of combined chromium oxide coating which provides an excellent substrate for subsequent lithographic coatings, varnishes, lacquers, enamels and other organic coatings. 7

Instead of applying the coating of the present invention to the can after it is manufactured or after it is filled and sealed, it is simpler to make the can from steel sheets that have the coating applied while they are being produced. 'This permits the coated sheets to be stored, if desired, and even shipped over substantial distances without requiring any supplemental treatment to prevent them from becoming unmarketable or unsightly as a result of corrosion.

The sheets are readily coated by first making sure they are very clean and then passing them through an aqueous solution of chromic acid and a reducing agent which is compatible with the chromic acid. This compatibility means that the chromic acid and the reducing agent, even though both are-present in the solution, will not in the 3,346,522 Patented Oct. 10, 1967 appropriate dilution react rapidly with each other and will not form a visible precipitate while the solution is being contacted with the steel.

The steel sheet Wet with the solution is then dried at a temperature above 212 F. During this drying, the chromic acid reacts with the reducing agent and becomes partially reduced. As a result, the final coating is a combination of hydrated chromium oxides containing about 20 to chromium by weight.

An inordinately high degree of corrosion resistance is obtained when 40 to by weight of the chromium inthe final coating is in what corresponds to the trivalent condition, the remainder being hexavalent, and the coating itself weighs about 10 to milligrams for every square foot of surface that it covers. Coatings of less weight give less protection, although coating weights need not be more than about 30 milligrams per square foot to give all the protection that is needed.

If the reducing agent is omitted from the coating bath, dried coatings of the above weight range will not show the desired trivalent chromium content. They also tend to be somewhat deliquescent and therefore inferior.

Typical reducing agents suitable for use in connection with the present invention are organic polyalcohols such as sugars, including invert sugar, sucrose, dextrose, glycol and polyethylene glycols, glycerine, mannitol, sorbitol, triethanolamine, hydroxylamine salts such as its sulfate and hydrochloride, phosphorous acid, and others such as the aluminum lactate mentioned in application Ser. No. 738,648. The preferred reducing agents are those like the polyalcohols which are oxidized by the chromic acid without leaving water-soluble reaction products. Phosphorous acid is oxidized tophosphoric acid by the chromic acid, and the phosphoric acid is converted to phosphates that are insoluble in water, by reacting with the surface of the coated metal.

The proportion of reducing agent to chromic acid should be insufficient for the complete reduction of all the chromium to trivalent condition. The minimum amount of reducing agent is somewhat below that which will in a body of solution reduce the lowest proportion of the chromium, inasmuch as some of the chromium is reduced during the drying operation even if the reducing agent is not present in the bath. Chromic acid solutions of almost any concentrationcan be used and the coating weight adjusted by controlling the amount of solution that is left on the metal surface when it is being dried. However, concentrations of from about /2 to about 30% CrO are most readily used.

The final heat treatment is somewhat more effective if carried out at temperatures substantially above 212 F. Temperatures between 275 F. and 450 F. are preferable and provide the most corrosion-resistant and adherent forms of coating. At about 500 F. and higher, however, the coating appears to be adversely affected unless the exposure to that temperature is held down to about two seconds or less.

The same coated sheet can be used for making both the top and bottom covers as well as the body of a can, although in some cases different thicknesses of metal can be used in the different portions, so that separate coated sheets are required.

The above coating can be readily applied either by a batch process or continuously. It may, for example, 'be added at the end of a standard sheet steel production line. In fact, the coating can be effected with the steel moving at a relatively high speed through the necessary treating stations. Under some conditions, particularly where the steel is moving very rapidly through a chromic acid bath, it is desirable to have a wetting agent present in the bath. This enables the bath liquid to more rapidly and uniformly wet the surface of the metal. Wetting agents of any type can be used so long as their wetting action is not completely destroyed by the oxidizing action of the bath. Anionic, cationic, or nonionic types of wetting agent used in amounts of about 0.001 to 0.2% by weight of the bath are effective to keep from developing coating irregularities apparently due to air bubbles trapped on the metal while moving through the chromic acid bath. A highly effective example of wetting agent is the polyoxyethylene ether of alkylated phenols such as that produced by condensing dodecyl phenol with twelve molecules of ethylene oxide. Reference is also made to US. Patents Nos. 1,970,578 and 2,085,706 for more specifically disclosed wetting agents that are suitable.

One example of the coating process of the present invention is as follows:

EXAMPLE I A. Sheets of 14 mil thick SAE 1010 steel are cleaned by immersing for five seconds in a 180 F. aqueous solution of disodium phosphate and 3% sodium earbonate.

B. The cleaned sheets are rinsed with water at 70 F.

C. Immerse the rinsed sheets in solution of sulfuric acid in water for one second at 70 F.

D. Rinse in water at 70 F.

E. Subject the resulting sheets to a uniform action of jets of an aqueous solution containing 1.2% nitric acid at 80 F. for two seconds, the jets impinging at a velocity of ten feet per second.

F. Rinse the acid treated sheets in cold water at 70 F.

G. Pass the sheets between rotating brushes having stainless steel bristles, to remove loose material.

H. The rinsed sheets are sprayed with an aqueous solution of 3 /2% chromic acid, 1% pentaethylene glycol and 0.1% of the Wetting agent made by condensing 3 mols of ethylene oxide with p-(n-octyl)phenol. This solution was prepared in the mixing nozzle by supplying it with two separate streams, one being an aqueous solution of the chromic acid, and the other an aqueous solution of the remaining ingredients.

I. The sheets carrying the above solution are passed through an air oven, the inside of which is held at 800 F., the surface of the sheets reaching a temperature of 375 F.

J. The hot sheets are quenched with water at 70 F., and

then permitted to dry.

The dried product is extremely resistant to corrosion, particularly if coated with an acrylate lacquer or even a thin film of methylmethacrylate resin. Other resins such as those made with ester-type waxes, carnauba wax for example, are also very effective.

The mixed chromium oxide coated steel of the present invention is further protected against corrosion by applying to the coating a film of an oil such as a paraffin or a glyceride oil. Thus mineral oil or palm oil can be applied in very minute quantities (0.5 milligram or more per square foot) as by conventional electrostatic coating techniques and enhances the corrosion resistance of the oxide coatings as well as reduces friction to simplify fabrication operations such as stamping, bending, etc.

Joints in cans or the like made of steel or other solderable metals coated with the mixed chromium oxides of the present invention, can be sealed by soldering as described in parent application Ser. No. 6,021.

Soldering is also a good way to seal joints in metal cans or the like where only one surface of the metal carries the mixed chromium oxide layer of the present invention. For example, a dog food can may have its internal surface tinned and its external surface coated with such oxide layer. The flux treatment described in Ser. No. 6,021 does not affect the tinned surface so that the attraction of the tin to solder helps in the soldering. The inside and outside surfaces of such a container can also be interchanged,

but in either case it is helpful to apply an organic coating over the tin and/ or the mixed chromium oxide coating, to improve their characteristics.

The soldering of container joints is helpful in that the solder covers and further protects the portions of the container walls that are crimped together to make the joints. Such crimping can be quite severe and can subject the metal to so much distortion as to fracture or loosen the mixed chromic oxide coating, particularly if the coating is relatively heavy. Soldering at a joint over such severely deformed metal will fully protect the metal against corrosion.

The coatings of the present invention can be applied to articles after they are formed as by shaping, stamping, or even casting. When treating such formed articles with the liquid coating solutions, however, care should be taken to see that the layer of solution on the article does not concentrate as by running or dripping to any portion of the article that should be protected. A localized thick layer of solution tends to form on the lower edges of the article under the influence of gravity, but a blast of air can be used to redistribute the thick layer. Alternatively, the articles can be rotated or kept moving so as to keep a thickened film from forming at any location.

The coating solutions for the present invention need not be prepared from separate chemicals which have to be carefully combined in the proper proportions by the final compounder. It is a feature of the present invention that the dry or concentrated chemicals can be mixed in the proper proportions and shipped as such a mixture for use in the coating operation. Such concentrated mixtures are more readily shipped since they take up less room and are not encumbered by large quantities of water. Furthermore, such dry mixtures containing CrO can be stored indefinitely and are much more stable than solutions.

This stability is an unexpected characteristic of mixtures of CrO and sucrose, for example. When these materials are mixed together in perfectly dry condition they show no tendency to react, notwithstanding the fact that when in dissolved form these two materials react with each other more and more violently as the concentrations of the solutions increase. The regular commercial grades of these materials have moisture contents less than 0.1% and can be directly mixed, and the resulting mixture placed in a conventionally sealed container such as capped bottles or even paint cans with friction covers. The mixing need not be such as to produce a uniform mixture, although uniformity is readily obtained and is desirable where the entire contents of a single container need not be used at one time. However, the mixing itself should be carried out under conditions of low relative humidity or with only limited exposure to high humidity in order to keep the CrO from absorbing too much moisture.

To use the prepared dry mixture for coating it is only necessary to drop the contents into a sufficiently large quantity of water preferably with stirring, so as toprevent local high concentrations that can cause reaction. As soon as the solids are completely dissolved, the solution is ready for use. If undissolved material is present in the solution, it is prefer-ably filtered before use.

Any of the above-mentioned water-soluble reducing agents that are solid will, when substituted for the sucrose, also be stable in the above mixtures and can be similarly used in conjunction with the present invention. All these reducing agents are compatible with a 4% water solution of CrO for at least one day at 'F., but react with the CrO in films of said solution at temperatures above 212 F. to cause reduction of from about 40 to of the chromium in the CrO to trivalent condition. The proportions of the ingredients can be from 3 to 5 parts by Weight of CrO as against from 1 to 2 parts by weight of the solid water-soluble reducing agent.

In accordance with the present invention the above dry mixtures can be further improved by including in them oxides or carbonates of metals such as zinc, magnesium, aluminum and calcium in a proportion up to that which is stoichiometrically required to form a dichromate with the CrO Such addition acts to dilute somewhat the dry concentration of the CrO and thereby further safeguards the stability of the dry mixture, should the mixture for example be inadvertently exposed to high humidity. Furthermore, these additions, particularly when sufficient to actually reach the stoichiometric proportion, greatly stabilize the. coating solution that is eventually formed from the dry mix, all without detracting from the etficacy of the final coatings.

As an example of the above dry mixes, there can be combined 3 parts by weight of CrO one part by weight of sucrose, and 1.2 parts by weight of ZnO. The CrO is in the form of a powder that just passes through a 150 mesh screen, the sucrose is in the form of ordinary granulated sugar as supplied to the grocery market, and the ZnO as a +250 mesh powder. The ZnO can first be mixed with the CrO after which the sucrose is added to give the final blend which can be stored for several years if necessary. Upon pouring into sufiicient water to provide a solution having 3% CrO by weight, an eflectlve coating solution is prepared. In some cases a small amount of normal zinc chromate, insoluble in the solution, is also formed and it is desirable to decant or filter the solution away from such residue.

Instead of the above dry mixtures, a pre-prepared dichromate can be mixed with the reducing agents of the present invention. Strontium dichromate is an example of such a dichromate, and can be readily prepared as coarse crystals about to 5 inch in size. In general, watersoluble dichromates of metals having a valence greater than 1, such as the dichromates of zinc, magnesium, calcium, strontium, and aluminum are suitable for this purpose and are in some respects more desirable since they can tolerate a somewhat higher moisture content than the mixtures with unreacted CrO and can in fact be mixed in atmospheres having a humidity of as much as 75% or more. It appears that these dichromates are less hygroscopic than CrO and not only is less precaution needed for the mixing, but the mixture need not be so carefully sealed.

The reducing agents can also be partially or completely protected from contact with the CrO or the dichromate. One simple way to do this is to introduce the ingredients into a container in such a manner that they do not mix to any appreciable degree. There will then be a fairly thin boundary layer between the ingredients where the only actual contact of the reactants will occur. The reducing agent particles can also be first thoroughly mixed with very fine powdery forms of the above zinc, magnesium or aluminum oxides, so that the particles tend to become coated by these oxide powders and thereby somewhat insulated from the CrO As described in applications Ser. No; 666,852 filed June 20, 1957, Ser. No. 708,772 filed Jan. 14, 1958, Ser. No. 738,648 filed May 29, 1958, and Ser. No. 814,200 filed May 19, 1959, the coatings of the .present invention can also contain hydrophobic resins and/or pigment. These ingredients are provided in aforrn in which they are dispersed in water so that they readily mix with additional water to form the final coating solution. Concentrated resin dispersions having as much as 50% resin solids by weight are available, and in these concentrated dispersions the reducing agent of the present invention can be dissolved to form a stable concentrate which has compactness features approaching those of pastes. Such a formulation can be readily dissolved in water to the required dilution, and then mixed with the necessary solution of CrO or dichromate to form a coating solution that applies a resin-containing coating as described in the last mentioned prior application. An illustration of such a concentrate is as follows:

6 EXAMPLE 11 One gallon of an aqueous hydrophobic resin dispersion made as described below and containing 46% of solids by Weight.

One gallon of a by weight solution of sucrose.

The two liquids are mixed and yield a viscous stable milky dispersion that can be kept indefinitely and will, after dilution to 4% solids weight and in combination with an equal volume of a 6% CrO solution in water added in the form of ZnCr O provide a coating bath that when applied with rubber-covered rolls gives a very elfective coating on AISI types 400 and 300 stainless steel, alumimum, and galvanized steel, with a 5 second cure at 400 F.

The above resin dispersion is made by mixing 3300 milliliters of deionized Water,

200 grams of para-normal octyl phenoxy octaethoxy ethanol,

1000 grams of inhibitor-free methyl methacrylate,

800 grams of inhibitor-free ethyl acrylate,

36 milliliters of a solution of 0.3 gram 'FeSO4-7H O (reagent grade) in 200 milliliters deionized water,

9 grams ammonium persulfate.

This mixture at room temperature is placed in a glass container and stirred while there is added 9 grams sodium metabisulfite, 2.5 grams t-butyl hydroperoxide.

Stirring is continued until the temperature subsides after an initial rapid heat evolution. The reaction mixture is then cooled to room temperature andto it is added another batch of the same ingredients, all but the t-butyl hydroperoxide being dissolved in a quantity of deionized water A of that used in the original batch. The t-butyl hydroperoxide is then added with stirring to cause the second batch to react and bring the resin content to about 46%.

Other resins such as methyl methacrylate homopoly- 'mers, the alkyds, butadiene-styrene copolymers, polystyrene, polyvinyl chloride and even polytetrafluoroethylene, can be used in place of the resin of Example II with similar results. In general the proportion of resin to reducing agent should be between about 1:2 and 6:1 by weight, and the concentrates have at least about 30% nonvolatile ingredients (essentially resin plus reducing agent) by weight. Specific examples of using such other resins are given in applications Ser. No. 708,772, Ser. No. 738,648

7 and Ser. No. 814,200.

It will be noted in this connection that the polytetrafl-uoroethylene resin does not form a uniform adherent resin film when used in accordance with the present invention, but that the final coating it makes can be wiped or brushed free of loose particles to leave a very light coating having very good corrosion resistance. Resins that are of a waxy nature, such as polytetrafluoroethylene or even polyethylene, do not provide a coating suitable for receiving a covering 'layer of paint, for example, particularly when present in the coating at a concentration of as low as 25%. However, the degree of corrosion resistance imparted by the resin-containing coatings Whether waxy or non-Waxy is so high that additional protective films are not needed. On the other hand, when hydrophilic resins, such as polyacrylic acid, are used, the corrosion protection is much poorer.

Pigments can also be incorporated in the above concentrated resin dispersions, or where resin-free mixtures are desired, the reducing agents can be incorporated in the pigment dispersions of the type referred to for instance in application Serial No. 814,200. Effective examples of such concentrates are:

EXAMPLE III 50cc. of a 50% by weight solution of sucrose in water,

285 cc. of an acrylate resin prepared as described in Example II but with a ratio of methyl methacrylate to ethyl acrylate of 1:4 by weight,

167 cc. of an aqueous dispersion of a mixture of 95% carbon black and phthalocyanine blue, the dispersion having a 25% solids content and stabilized with paranormal octyl phenoxy triethoxy ethanol in a concentration of 0.05%.

The three ingredients are mixed together to form a stable dispersion that will keep without any of the ingredients settling out. Before use it can be first mixed with 100 cc. of an aqueous solution of zinc dichr-omate having a concentration of 1.05 grams zinc dichromate per cc. and then diluted to 1 liter with water. The resulting mixture forms a very effective black coating on bare steel either plain or pre-etched with nitric acid, it the mixture is rolled onto the steel with rubber rolls and the thus coated steel heated with a gas flame to 400 F. for one and one-half seconds.

EXAMPLE IV 300 cc. of the acrylate resin dispersion similar to that of Example 11, sold by Rohm & Haas, Philadelphia, Pennsylvania, under its designation Rhoplex C-72,

30 cc. of an aqueous solution of para-normal octyl phenoxy tetra-ethoxy ethanol having a concentration of 20% by Weight,

100 cc. of a 50% by weight aqueous solution of sucrose,

200 cc. of an aqueous dispersion of phthalocyanine blue containing 25.6% pigment by weight and stabilized with 0.1% of the above paranormal octyl phenoxy tetraethoxy ethanol,

54 cc. of an aqueous dispersion of titanium dioxide containing 36% TiO by weight and 3.3%' hydroxy ethyl cellulose.

The resulting mixture can be stored for indefinite periods although after some time some of the titanium dioxide will settle out. The settled material r-edisperses itself readily upon stirring.

Before coating there can be added to the mixture 300 cc. of an aqueous solution of zinc dichromate having 0.7 gram of the dichromate per milliliter, and it is then ready for application by a conventional spray gun. Aluminum construction panels so sprayed and then cured by holding for ten seconds in an air oven kept at 1000 F., show better outdoor resistance than anodized colored aluminum.

Straight chromium steel, stainless steel, aluminized steel, galvanized steel and bare steel panels similarly coated are very satisfactory surface panels for building construction. Other pigments such as iron oxide reds, chrome yellows and chrome oxide pigments and the like, also can be used in place of the pigments of the above examples, or can be combined with other pigments to give varying colors.

Another pigment-free formula is as follows:

EXAMPLE V 1 gallon of Teflon dispersion prepared in accordance with Example Ill of U.S. Patent 2,478,229 and concentrated to about 55% solids by weight,

1 gallon of 50% sucrose by weight,

30 grams of para-normal octyl phenoxy hydroxy ethoxy ethanol.

The above mixture is stable and can be stored for many months. For use it can be diluted with twenty times its volume of water and the diluted material mixed with a 4 /2 solution of zinc dichromate in water. A zinc base die casting having a chromium plating over an intermediate nickel plating which is in turn over a copper plating, when dipped in the resulting solution and heated in an oven held at 390 F. until the metal reaches oven temperature, leaves a whitish film. Wiping off the film with a cloth leaves an almost invisible residual coating layer that weighs about 3 to 6 milligrams per square foot and increases tremendously the corrosion resistance of the plated metal.

Some reducing agents such as those which are ionizable salts, tend to render dispersions unstable and are better avoided in the making of concentrated resinan'd/ or pi ment-cont-aining dispersions. The nonionic polyalcohols such as sucrose, glycerine and the like do not show this destabilizing tendency.

The Wetting agents used in the above examples are very effective in keeping the resin and/or pigment particles dispersed, but they can be replaced by other nonionic, anionic or cationic wetting or suspending agents without affecting the corrosion resistance of the final coat- Anhydrous coating mixtures of a hexavalent chromium compound with a reducing agent can also be prepared in relatively stable form with such reducing agents as polyethylene glycols having molecular weights of at least about 4000. These glycols are unctuous or waxy in nature and accordingly are much more intimately in contact with the hexavalent chromium compound with which it can be uniformly or nonuniformly mixed. While this intimate contact generally makes for a poor stability, these glycols are so resistant to oxidation at temperatures as high as 225 F., that the mixtures can be stored for many months at 180 F. Without loss of usefulness.

One example of such a long-lived mixture is prepared by mulling 3.5 parts by weight of Zinc dichromate powder with 1 part by weight of polyethylene glycol having a molecular weight of 8,000. CrO can also be used in place of the zinc dichromate.

The resin-free and resin-containing coatings described above can also be applied by a tumbling barrel technique to objects that are small enough to be so treated. By way of example, small stampings for snap switches can be tumbled in a stainless steel basket while the parts are flooded with an aqueous solution of 4% zinc dichrom'ate and 1% sucrose. The excess solution is permitted to drain off while the tumbling is continued, and with the tumbling still continuing, a stream of unheated air is blown through the basket. This is continued until all the parts are dry, after which they can be stoved to bring their temperature to 350 F. for best results. The parts are now ready for assembly into the switches. Plain carbon steel, alloy steels, stainless steels, brass, copper, aluminum, and in general all metals can be so coated. Heated air can also be used in place of unheated air if desired.

An efficient tumbling basket for the above purpose is made of 188 stainless steel wire 5 inch thick spaced A1. inch apart, although other dimensions can be used for other types of work articles. Nylon-coated plain carbon steel wire can also be used in place of the stainless steel to make the basket. Ordinary perforated tumbling drums will work, but are somewhat less suitable.

Other coating solutions such as the dichromate-free CrO solutions and the resin-containing solutions can also be applied in the same manner, and screws, rivets, nails and the like can be coated by the tumbling technique.

Such objects as steel fuel lines for automobiles and the internal as well as external components of water meters, are also desirably coated with the resin-free or resin-containing coatings of thepresent invention. These fuel lines generally have an inside surface of bare steel which can become corroded by moisture present in the fuel. For treating these lines, the coating mixture of the present invention is introduced into the line after which a somewhat loosely fitted plug is forced through the line as byair pressure, to wipe off the excess solution. The so-treated line is then subjected tothe high temperature operation. Where the plug is fitted too tightly, the concentration of the coating solution should be very high inasmuch as only very thin films of the solution will be left on the tubing wall.

In connection with the water meter use, the internal components of the meters are very effectively protected against corrosion for considerable periods of time even though these components remain immersed in water continuously during use. Such internal components made of stainless steel will also have their useful life greatly extended by such coating treatment.

A feature of the present invention is that unusually protective coatings can be applied from a water base mixture and in a single application. Fire hazards associated with organic solvents are completely avoided and the one-shot application is particularly desirable inasmuch as the curing step requires such a short treatment that it can be carried out on a rapidly moving metal treating or metal producing line without calling for inordinately long ovens.

The proportion of chromium that is reduced to trivalent form in the coating of the present invention is preferably at least 70% of the total chromium present. Lower conversions leave the final coating with a brownish appearance and also do not give the best corrosion resistance. When the reducing agent is decreased in amount so that the chromium conversion drops below 40%, the curing time is increased and begins to become a serious obstacle. For instance, the complete elimination of a reducing agent from the resin-dichromate coating formulations produces coatings that are sensitive to moisture and do not provide suflicient protection unless the curing is carried out for at least five minutes at a minimum temperature of 350 F. Even then the chromium conversion is about 30% with a resin-to-dichromate ratio of about 2:1 only the CrO content of the dichromate being considered, and the conversion is even less with lower proportions of resins.

Such inadequately converted coatings are greatly improved by a hot water wash, apparently because some unconverted material is dissolved out. For instance, a product coated as in the above Example IV and cured for ten minutes at 325 F., develops better corrosion resistance and a more brilliant color if washed for one minute with 180 F. tap water. After such washing the chromium conversion analyzes at about 58%. For this improvement the washings should bring the conversion to at least 50% and generally at least a one-half minute treatment with water 160 F. or hotter is needed. The same improvement is brought about with all the resins and pigments and in coatings containing neither.

The coating formulations of the present invention are especially effective when the hexavalent chromium is in the form of zinc dichromate. This dichromate is inexpensive as well as very soluble in water and at the same time it shows an exceedingly low tendency to oxidize reducing agents when in water solution. It is accordingly the preferred form of hexavalent chromium.

The protective nature of the above coatings, whether or not containing resin, is also of value in extending the life of scalpels, razor blades, and the like. By way of illustration, a scalpel made of plain carbon razor steel when carrying a resin-free cured coating deposited from a solution of 1% chromic acid and /a% sugar, can be used for as many as five separate operations before it need be discarded. A similar razor blade also has a twoto fourfold increase in life when used for normal shaving. Such results are also obtained when the coating is deposited from a solution that, in addition to the chromic acid, contains resin in the proportions of the present invention, and the resins can be either thermoplastic or thermosetting. It appears that the life of a razor edge is determined to a large extent by corrosion, particularly where the edge is only used during widely spaced intervals of time.

Aluminum-surfaced and stainless steel building panels, storm windows, etc., that are exposed to weather are also very desirably coated in the above manner. The coatings greatly reduce pitting and corrosion whether or not the resin is included with the chrornic acid or dichromate, and whether the resin is thermoplastic or thermosetting.

It is a feature of the above-described resin-free coatings of in situ formed mixed chromium oxides, that they behave as though they are electrically conductive. One example of such behavior is that electroplatings can be applied over the coatings. Chromium, for instance, can be readily deposited at room temperature from an aqueous solution of 400 grams of chromic acid and 4 grams H 30 per liter, the coated metal being connected as a cathode, and using a cathode current density of 50 amperes per square foot. The current density can range from 20 to amperes per square foot, and the temperature can range from about 60 to F. The chromium deposits as a dull layer that adheres to the oxide coating, but can be removed by electrolytic stripping, using the chromium plating as an anode in an aqueous solution of 5% sodium hydroxide. After stripping, the mixed chromium oxide coating of the present invention is found still adhering to the base metal.

Copper, nickel, zinc and the other metals can also be electroplated over the coating of the present invention, in place of or in addition to the chromium, and any or all of these metals can be so plated on the mixed chromium oxide coating regardless of the base metal which carries the coating.

The in situ formed mixed chromium oxide coatings are also highly suited for protecting against corrosion such surfaces as those of drawn plain carbon steel cans, as used for example to hold aqueous mixtures such as aerosol type self-lathering shaving creams. Typical formulations for such a shaving cream are given in US. Patent 2,655,480 granted Oct. 31, 1953. Plain carbon steel containers for aqueous paint emulsions are also protected very effectively with such coatings. A preliminary grain boundary etch of the steel surface is helpful in increasing the protection, as is the incorporation of resin in the coating, but either or both of these refinements can be omitted and still have a very desirable product.

Most containers of the above or other types have their inside Walls subjected to sharply different conditions as compared to their outside walls. It is therefore desirable in such cases to coat the different faces of the metal in different ways. Accordingly, the resin can be omitted from the coating on the outer surface of the container Wall where the containers are not exposed to severe c0rroresin-free coating on the internal surface of such a container can be covered by the usual sanitary enamel or other paint.

The resin-chromium oxide coating being more resistant to acids and neutral detergents than the resin-free chromium oxide coating, the resin-containing coating can be used without a top coat of enamel when packing these materials. Oil-in-water'emulsion paints having an acidic nature are also desirably packed in the resin-chromium oxide coated containers.

Dissimilar coatings on the opposite faces are con veniently prepared by separately applying the appropriate dispersions to these faces of the original metal sheet, by means of transfer rolls. In other words, adjacent each face there can be a separate pair of rolls with different dispersions poured into the bight of-the rolls on each side. One of each pair of rolls is also arranged to contact the metal surface. By adjusting the spacing and/ or compression between each pair of rollers, the amount of coating dispersion carried through the bight is controlled and a uniform layer of the resulting film of coating dispersion is transferred to the metal surface. The rolls should be arranged to rotate and be fed with coating dispersion in such a manner. that the transferroll does not contact the metal surface until after it has received the dispersion and after the dispersion is passed through the bight. Identical or different coatings can be applied to the opposite faces of sheets while the width of the sheet is disposed either horizontally, vertically, or in any angle in between. With different coatings on the opposite faces, any runoff from the bights of the roll pairs should be kept from mixing and can be separately recirculated. With the sheet held at any desired angle, the path of movement of the sheet can be also selected from anything ranging from horizontal to vertical.

Better Wetting of the metal is obtained by incorporating in the coating formulation a quantity of wetting agent or of organic solvents such as tertiary butyl alcohol, acetone or similar materials that are driven off during the curing step yet tend to reduce water-breaks when the metal surface is not perfectly clean.

In the coating of continuous sheets the use of liquid quenching is desirable apart from the improved corrosion resistance it contributes. When a coil of such sheet emerges from the curing oven of a coating sequence, the metal might not cool rapidly enough to permit immediate recoiling. Such recoiling is best carried out with the metal no hotter than about 175 F. Liquid quenching will then immediately bring the metal down to recoiling temperature. Water-cooled rolls can also be used to cool down the metal, and forced air drafts are likewise helpful, but they do not contribute the extra corrosion resistance, and do not supply the small amount of free chromic acid that can be conveniently added to a quenching liquid.

The use of a quenching liquid also provides a simple way to increase the conversion ratio of the chromium in the coating, as described above. For example, in an extreme situation where as originally cured the conversion is only a half-minute wash with 165 F. Water will increase the conversion to 40%. Hotter water will increase the conversion in shorter times.

A particularly preferred technique for providing an unsymmetrically coated metal sheet is to take a corrodible ferrous metal sheet such as black plate, subject both surfaces of the sheet metal to a grain boundary etch, electrolytically deposit a tin plating on one surface of the etched sheet, applying to the other surface an aqueous dispersion of (a) from /2 to of a compound selected from the class consisting of chromium trioxide and watersoluble dichromates of metals having a valence greater than 1, and (b) a reducing agent compatible with said compound for at least one day at 803i, but effective to reduce from about to about 95% of thechromium in said compound to trivalent condition When heated to 275 F., then evaporating thewater from the dispersion and heating the resulting coated sheet metal to a temperature high enough to fuse the tin plating and cure the coating on the other surface, but not high enough to damage the last-mentioned coating.

One effective procedure for such an unsymmetrically coated sheet is as follows:

EXAMPLE VI Stannous sulfate 54 Sulfuric acid 100 Monosulfonated ortho cresol 100 Beta-naphthol 1 Gelatin u 2 The remainder being water.

The bath is kept at 75 F. and a pure tin anode is used and located opposite one face of the sheet. The anode current density is 12.5 amperes per square foot 12 and the cathode density is 25 amperes per square foot, the plating time being about 30 seconds so as to deposit a plating of about /2 pound per base box, essentially entirely on the surface facing the anode.

The plated sheet is then rinsed in Water at 70 F. and its opposite face is then subjected to the coating of Example III, after which the sheet is passed through an air oven, the inside of which is held at 1000 F., the steel sheet reaching a temperature of 450 F. The tin layer is fused at this temperature to convert it into the shiny condition normally used, and at the same time the coating on the opposite face is cured by a conversion of of its hexavalent chromium to trivalent condition. It can then have its tin layer subjected to a palm oil treatment, as in the usual tin plating technique.

The above procedure can be varied by making the grain boundary etch more or less intense. Such an etch that removes more than 400 milligrams per square foot is not desirable unless the in situ formed mixed chromium oxide coating is relatively heavy, that is above about milligrams per square foot. Also grain boundary etches that remove less than about 50 milligrams per square foot are of relatively little value. The grain boundary etch can be entirely eliminated with a corresponding drop in effectiveness.

The electrolytic tin plating can be deposited in any other way, such as by those described in pages 319 through 339 of Modern Electroplating published 1942 by the Electrochemical Society, Inc., New York, New York. Also the in situ formed mixed chromium oxide layer can be of any other type disclosed in the above-mentioned prior patent applications as well as prior patents No. 2,768,103, No. 2,768,104, No. 2,773,623, No. 2,777,785 and No. 2,861,906.

The latter coatings can in general be deposited from aqueous dispersions containing from /2 to 30% of a compound selected from the class consisting of chromium trioxide and water-soluble dichromates of metals having a valence greater than 1, and a reducing agent compatible with said compound for at least one day at 80 F., but effective to reduce from about 40 to about 95 of the chromium in said compound to trivalent condition when heated to at least 275 F.

The resin-containing and pigment-containing coating formulations are more suitable for application by spraying than the straight coating solutions containing neither resin nor pigment. This may be attributable to the fact that solutions are more difficult to apply properly in this way than dispersions. The dispersions can very easily be used to apply in a single sprayed-on coat a cured coating layer as thick as 4 grams per square foot. For best results the spraying is carried out with an air stream arranged to evaporate a large fraction of the dispersion liquid even before it reaches the surface being coated. Even heavier coatings can be sprayed on by heating the surface being coated to a temperature of -200 F. This causes more liquid evaporation so that the deposited layer is not apt to run. Very uniform thick coatings are applied in this way.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed:

1. A concentrate for mixing with aqueous solutions of hexavalent chromium coating compounds, said concentrate being an aqueous solution of sucrose in which is dispersed an acrylic resin, the resin and sucrose, being in the proportion of between about 1:2 and 6:1 by weight, and together constituting at least about 30% by weight of the concentrate.

2. A concentrate for mixing with aqueous solutions of hexavalent chromium coating compounds, said concentrate being an aqueous solution of sucrose in which is dis- 13 14 persed a butadiene-styrene resin, the resin and sucrose 2,775,535 12/1956 Poole. being in the proportion of between about 1:2 and 6:1 2,777,785 1/ 1957 Schuster et a1. 148--6.2 by weight, and together constituting at least about 30% 2,798,020 7/1957 Balz et a1. by weight of the concentrate. 2,954,358 9/1960 Hurwitz 26029 3. The combination of claim 1 in which the concen- 5 2,961,340 11/1960 Meier 117-145 X trate also contains dispersed pigment. 2,974,091 3/ 1961 Neish 20437.5 3,053,693 9/1962 Schuster et a1 11775 References Cited PATENTS UNITED STATES PATENTS FOREIGN 104,510 8/1917 Great Britain.

2,066,180 12/1936 King.

2,472,592 6/1949 Kiefer 1486.16 X RICHARD D, NEVIUS, Examineih 2,595,952 5/1952 Kunze et a1. 260-29.6 2 4 919 7 54 Berry et a1. 17 145 I. R. BATIEN, JR., ASSISZKZHZ Exwmmer. 2,721,150 10/ 1955 Grantham 171 15 ALFRED L. LEAVITT, Primary Examiner.

2,748,549 6/1956 Tuttle. 

1. A CONCENTRATE FOR MIXING THE AQUEOUS SOLUTIONS OF HEXAVALENT CHROMIUM COATING COMPOUNDS, SAID CONCENTRATE BEING AN AQUEOUS SOLUTION OF SURCROSE IN WHICH IS DISPERSED AN ACRYLIC RESIN, THE RESIN AND SUCROSE BEING IN THE PROPORTION OF BETWEEN ABOUT 1:2 AND 6:1 BY WEIGHT, AND TOGETHER CONSTITUTING AT LEAST ABOUT 30% BY WEIGHT OF THE CONCENTRATE. 